Spectroscopy of dimers, trimers and larger clusters of linear molecules

Rotationally resolved spectra of van der Waals clusters provide the most direct, precise and unambiguous experimental probe of intermolecular potential energy surfaces, particularly in the lower energy range of attractive forces. The spectroscopy of such weakly bound complexes is reviewed with a focus on those containing only the following five small linear molecules: N2O, CO2, OCS, CS2 and C2H2. Even with this limitation, there are over 30 such complexes for which high-resolution spectroscopic results are now available in the microwave, millimetre wave and infrared regions. Dimers formed from these five species tend to be planar with slipped parallel structures, though there are some exceptions. Trimers tend to be barrel-shaped, that is, non-planar with approximately parallel monomers. Larger clusters are almost invariably non-planar and sometimes highly symmetric. Three significant themes emerge here. First, the possibility of determining low-frequency intermolecular vibrational modes by means of mid-infrared combination bands. Second, the increasing number of complexes for which more than one structural form, or isomer, is observed. And finally, the growing amenability of even large' clusters (say, tetramers and larger) to spectroscopic study. All three factors help to probe potential energy surfaces more completely in regions away from the global minimum.